Electrochemical fuel cells convert a fuel (e.g., substantially pure hydrogen, methanol reformate or natural gas reformate, or a methanol-containing stream) and an oxidant (e.g., substantially pure oxygen, oxygen-containing air, or oxygen in a carrier gas) to electricity and reaction product. Two or more fuel cells may be connected electrically in series to increase the overall power output of a fuel cell system. Such a multiple fuel cell arrangement is referred to as a fuel cell stack. The stack typically includes inlet ports and manifolds for directing the fuel stream and the oxidant stream to the individual fuel cell reactant flow passages. The stack also may include an inlet port and a manifold for directing a coolant fluid (e.g., water) stream to interior passages within the stack to absorb heat generated by the fuel cell during operation.
Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly that includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes. The electrodes typically are formed from a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The membrane electrode assembly contains a catalyst (e.g., platinum powder) at each membrane/electrode interface to induce a selected electrochemical reaction. In operation, the electrodes are connected electrically by an external electric circuit. The fuel moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. The oxidant, on the other hand, moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer to form a reaction product. In hydrogen based proton exchange membrane fuel cell systems, the electro-catalyzed reaction at the anode produces protons and electrons. In direct methanol fuel cells, methanol and water react to form carbon dioxide, protons, and electrons. The ion exchange membrane enables the flow of protons from the anode to the cathode. The membrane substantially separates the fuel stream from the oxidant stream. At the cathode electrocatalyst layer, oxidant reacts with the protons that have crossed the membrane barrier to form water as the reaction product. Product water formed at the cathode electrode may be removed by evaporation or entrainment into a circulating gaseous stream of oxidant, or by capillary action into and through a porous fluid transport layer adjacent to the cathode.
Water management has become crucial in the development of fuel cells, including direct alcohol fuel cells. Water may serve one or more functions within a fuel cell, including hydrating the electrolyte (e.g., a solid polymer electrolyte), diluting the fuel (e.g., to reduce fuel crossover), and serving as a reactant (e.g., methanol electro-oxidation). In a direct methanol fuel cell, for example, the anode requires water as a reactant and a fuel diluent; water collects at the cathode from diffusion and from electro-osmotic drag through the membrane. In an effort to decrease the overall volume and weight of a fuel cell system, it is advantageous to recycle water from the cathode. For cells with a sulfonic acid polymer electrolyte, an aqueous solution with less than 7% methanol by weight typically is needed at the anode for optimal efficiency. Thus, once such a fuel cell is started, using recycled water may eliminate 93% or more of the fuel storage weight.
Water recycling schemes for fuel cell systems have been proposed (see, e.g., U.S. Pat. No. 6,303,244 and International Publication No. WO 02/07241). In general, these approaches involve the use of pumps to transfer product water from the cathode to the anode.